The invention relates to an impact dot printer, and more particularly, to an impact dot printer including a print head having a plurality of driving solenoid coils for causing a plurality of print wires to impact a platen.
Generally, a printing wire of a wire impact dot head is driven utilizing the electromagnetic force of a solenoid coil. The solenoid coil is mounted in a magnetic frame and a terminal part thereof is soldered to a printed board through a hole provided on the bottom face of the frame. Since the frame is magnetic and is electrically conductive, a spacer made from a non-conductive material, such as plastic, is provided between the frame and the printed circuit board to prevent a short-circuit between the terminal part of the coil and the printed circuit board and frame. A heat radiating member is included to contact the peripheral edge of the frame. Heat generated by the solenoid coils is transferred from the magnetic core which contacts the solenoid coils to the peripheral edge of the frame and is then transferred to the radiating member which dissipates the heat into the air. In addition, in order to transfer the heat from the solenoid coil to the peripheral edge of the frame, a heat conducting resin such as silicone is injected between the frame and the coil. Alternatively, the coils can be directly soldered to the frame and the printed circuit board, negating the need for resin. The silicone is in liquid form when injected and solidifies after injection via natural or ultraviolet hardening in air.
The aforementioned structure is disadvantageous if low viscosity silicone is injected between the frame and coils because it can leak between the spacer and the printed circuit board before hardening. As a result, the silicone cannot be retained at the proper locations. Furthermore, any silicone which leaks must be removed during assembly. Accordingly, highly viscous silicone of more than 450 poise must be used for this application.
A print head 136 constructed and arranged in accordance with the prior art is illustrated in FIG. 1 and illustrates the difficulties encountered with this construction. Print head 136 includes a nose 101 coupled to a frame 102 with a plurality of solenoid coils 116 wound about coil bobbins 117 and positioned within frame 102. A printed circuit board 109 is disposed between frame 102 and nose 101 for electrically connecting coils 116 to a source of print signals. A spacer 108 is disposed between frame 102 and printed circuit board 109.
A plurality of levers 118 are mounted in frame 102 with a print wire 107 at the free end of each lever 118. Wires 107 extend through nose 101. Printed circuit board 109 is soldered to a coil terminal pin 112 to provide the electrical connection between printed circuit board 109 in order to control print head 136. When solenoid coils 116 are selectively energized, levers 118 are attracted to core 111 to drive wire 107 out of nose 101 to impact on a print medium. Spacer 108 is a thin plastic member disposed between printed circuit board 109 and frame 102 to avoid short-circuiting between solenoid coil 116 and frame 2 which would degrade performance. However, problems arise if spacer 108 is thin because solder is able to flow from printed circuit board 109 to frame 102 during assembly causing a short-circuit.
When spacer 108 is thickened in order to prevent solder flow and short-circuit, another problem arises. A projection 124 formed on nose 101 which joins frame 102 to nose 101 through an aperture must be lengthened in order to provide a proper fit. This is a difficult process because nose 101 is generally manufactured by injection molding or die casting. Thus, if projection 124 is significantly lengthened, it becomes weak and tends to break during assembly.
If a resin, such as silicone is not utilized, there is a limit to the amount of heat which can be effectively radiated for suppressing the temperature increase in the conventional printing head due to the heat generated over time when heat is transferred from the solenoid coil to the core and is radiated at the peripheral edge of the frame through the radiating member. Accordingly, it has been difficult to increase printing speed because the solenoid coils can burn out due to the temperature rise in the head. This in turn degrades the printing quality and life of the print head.
Referring to FIGS. 1, 2 and 3, if high viscosity silicone 120 is injected into print head 136, the heat generated by solenoid coil 116 cannot be fully released during high print duty. Because silicone 120 is viscous, it cannot reach a bottom face 110 of frame 102, the lower part of solenoid coil 116 or a protrusion part 121 of coil bobbin 117 as shown in FIG. 3. In addition, as shown in FIG. 3, resin 120 barely flows between adjoining solenoid coils 116.
Another problem arises in that the lower part of solenoid coil 116 and coil terminal pin 112 are fixed to printed circuit board 109 by solder. These components are influenced by vibration generated when print wire 107 is driven during printing. Where there is a winding sag or slack exists on coil 116, the vibration of print head 136 during printing causes friction between the slack wire portion and frame 102 causing the wire insulation to wear off. This in turn results in a short-circuit between solenoid coil 116 and frame 102.
Accordingly, it is desired to provide an impact dot printer having a wire impact dot print head which overcomes these problems encountered in the prior art and which provides a highly reliable print head which avoids short-circuit between the solenoid coil and the frame. The printer should be one in which heat dissipation and electrical integrity is increased while maintaining a structure which is durable and easily allows accurate positioning of the nose and frame during assembly.